High toughness alloy ateel with improved weldability

ABSTRACT

AN ALLOY STEEL HAVING 0.2% OFFSET YIELD STRENGHTS GREATER THAN 100K S.I. IN PLATES UP TO 6 INCHES THICK, HIGH TOUGHNESS AT TEMPERATURES DOWN TO -120*F., AND A VERY LOW TENDENCY TOWARD HAZ CRACKING. THE PRINCIPAL CONSTITUENTS OF THE STEEL COME EASILY WITHIN THE COLLOWING RANGES:   ELEMENTS PERCENTAGE BY WEIGHT C 0.09-.013 MN 0.60-0.85 P (MAX.) 0.015 S (MAX.) 0.020 SI 0.20-0.35 NI 2.5-4.5 CR 0.40-1.20 MO 0.25-0.60 V 0.05-0.10 AL 0.015-0.035   BALANCE ESSENTIALLY IRON.

United States Patent 3,759,706 HIGH-TOUGHNESS ALLOY STEEL WITH IMPROVED WELDABILITY Brian Mravic and Lew F. Porter, Monroeville, Pa., as-

signors to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Feb. 23, 1972, Ser. No. 228,738 Int. Cl. C22c 37/10, 39/20 U.S. Cl. 75-124 2 Claims ABSTRACT OF THE DISCLOSURE An alloy steel having 0.2% offset yield strengths greater than 100K s.i. in plates up to 6 inches thick, high toughness at temperatures down to 120 F., and a very low tendency toward HAZ cracking. The principal constituents of the steel come essentially within the following ranges:

Balance essentially iron.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a low carbon alloy steel and, in particular, to a high toughness alloy steel having improved weldability properties.

Description of the prior art There is a strong need for steels with yield strengths over 100K s.i., high notch toughness, and good weldability. For several years, a steel under military specification MIL-S-162l6G has been used by the Department of Defense. This steel exhibits a yield strength of 100 to 115K s.i., and a Charpy V-notch energy absorption of over 30 ft.-lb. at 120 F. in plates up to 3 inches thick. The composition of the steel in weight percent is: up to 0.20 C, 0110-0140 Mn, up to 0.025 P, up to 0.025 S, 0.15- 0.35 Ni, 1.00-1.80 Cr, 0.20-0.60 Mo, up to 0.02 Ti, up to 0.03 V, and up to 0.25 Cu. Typically, the carbon content is about 0.l8 percent. The microstructure of the steel in the as-quenched condition is essentially martensitc in plates up to 3 inches thick.

In various applications of the military specification steel, weldability problems have been encountered. Specifically, the steel is highly susceptible to cold cracking in the heat-affected zone adjacent to welds. It is well known that such cold cracking is directly related to the steel composition. One formula which relates the amount of heat-aifected-zone (HAZ) cracking in a cruciform test to steel composition is:

(Equation 1) Percent cracking=-105 +449 (percent C) +23.8 (percent Mn) +7.54

(percent Ni) +15.0 (Percent Cr) The equation predicts that molybdenum and vanadium have a statistically insignificant effect upon cracking. Based upon this formula, the steel under military specification MIL-S-162l6G would be expected to exhibit up to 48% cracking.

3,759,706 Patented Sept. 18, 1973 Most steels with high toughness and with yield strengths over K s.i. have a martensitic microstructure in the as-quenched condition. This is because it is generally believed that a fully martensitic as-quenched structure is necessary to insure high Charpy V-notch shelf energy and low transition temperature. To insure a fully martensitic microstructure in thick sections, either the carbon content, the alloy content, or both must be high. However, the high carbon and alloy content generally result in a strong tendency toward HAZ cracking.

SUMMARY OF THE INVENTION In accordance with the present invention, an alloy steel is provided which exhibits a high Charpy V-notch shelf energy and a low transition temperature. Its asquenched microstructure contains significant amounts of bainite thereby allowing a substantially lower carbon content. The combination of low carbon content along with moderate alloy content results in a low tendency toward HAZ cracking and a superior steel for producing various types of structural weldments for use as submarine hulls, ship hulls, tanks, etc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The improved steel of the present invention results from utilizing select ingredients in the following proportions:

TABLE I Element: Weight percent Carbon 0.09-0.13 Manganese 0 60-085 Phosphorus 0.015 Sulfur 0.020 Silicon 0 20-035 Nickel 2.5-4.5

Chromium 0.40-1.20 Molybdenum 0.25-0.60 Vanadium 0.05-0. 10 Aluminum 0015-0035 Balance iron and residual amounts of other elements.

The aluminum content listed is total aluminum (insoluble aluminum generally comprises less than 0.007%).

In order to attain optimum properties with a minimum required total alloy content and maximum weldability, the composition should be varied according to the thickness of the section to be produced. For example, for waterquenched plates up to 1% inches thick, the nickel, chromium, and molybdenum contents should be: 2.5-3.0 Ni, 0.4- 0.6 Cr, and 0.25-0.35 Mo. For plates from /2 to 4 inches thick, the nickel, chromium, and molybdenum contents should be: 3.0-4.5 Ni, 0.80-1.0 Cr, 0.35-0.45 Mo. For plates from 4 to 6 inches thick, the nickel, chromium, and molybdenum contents should be: 4.0-4.5 'Ni, 0.9-1.2 Cr, and 045-0160 Mo. For each of the three ranges of plate thickness, the other components should be in the ranges previously specified.

As previously mentioned, it is commonly believed necessary to have a fully martensitic microstructure to insure high Charpy V-notch shelf energy and low transition temperature. The present invention dispells this belief by providing an as-quenched microstructure that is generally a mixture of martensite and bainitedepending on the plate thickness and specific composition. By thus eliminating the requirement of a fully martensitic as-quenched structure, the carbon content can be lowered significantly without necessitating a concomitant increase in alloy content. As such, a tough, high strength steel can be obtained over a wide range of thicknesses with the advantage of substantially minimizing HAZ (heat-affected-zone) cracking.

Table II is included to illustrate the improved weld- The heat was vacuum-degassed in a ladle-to-ladle stream degassing unit prior to teeming; Ingots were formed and rolled into 1, 1 /2, 2, 3, and 4 inch thick plates by cross rolling to minimize anisotropy. Table IV 5 shows the slab and plate sizes produced from four ingots as well as the rolling ratio for'each of the plates.

ability of the alloy steels of the present invention. The HAZ cracking formula (Equation 1) was applied to representative prior art specimens and compared to the steel alloy of the present invention.

TABLE II.-PREDICTED HEAT-AFFECTED-ZONE CRACK- ING FOR TYPICAL MILITARY SPECIFICATION STEELS OF THE ALLOY STEEL OF THE PRESENT 1O Composition, percent Predicted HAZ cracking Cr M0 TABLE IV.-PROVISIONING FOR BO-TON ELECTRIC FURNACE HEAT 91 5120 4 4%454324 0 0 0 0 QQQ00 8 84.60 ewwwmei LLLLLLLLO 299 75036 999%96633 1 9986 3%%%2232 0 0 0 0 Q0 0 0 0 8 7 immummmi Present inventionl h m 2345678 3 S Ingot 2 10 by 97-.. 14, 450 2 by 104 by 225--.. 10 2 by 1 m n 6 e W it h r m m hem 6P n t t m WWa mbum o m I t ore? m w Mwa aeT S de C Bd e n MmW CmM fi u wmw d r t a W 0 O wn cp mm .I I U 6.1 P On nfl e k anm.m se n C A me m f n0 HPmo mp TABLE III Mn Total...- 28,900 1 Transverse to longitudinal ingot-to-plate rolling ratio (relative to w m a t 0 g n .i 0 5 4 4 5 860197 0 23 402 .0030000 00 O n n u -O -lr 1 PSSNCMVA Table V shows the Heat Treatment given the various plates.

TABLE V.HEAT TREATMENT OF STEEL PLATES FROM 80-TON FURNACE HEAT Austenitizing procedure First Tampering procedure First Second Second Plate 5, Temp., Time, Temp., Time, Temp., inches Time, Temp., Time, F. min. F. min

Plate thicknes No.

0.13 percent carbon, from 0.60 to 0.85 percent manganese, less than 0.015 percent phosphorus, less than 0.020 percent sulfur, from 0.20 to 0.35 percent silicon, from 2.5 to 4.5 percent nickel, from 0.40 to 1.20 percent chromium, from 0.25 to 0.60 percent molybdenum, from 0.05 to 0.10 percent vanadium, from 0.015 to 0.035 percent aluminum, and the balance essentially iron.

2. The steel of claim 1 characterized as having an as-quenched microstructure of both bainite and marten- 10 site.

TABLE VI.-TENSILE AND IMPACT PROPERTIES OF MILL-HEAT-TREATED PLATES OF THE 8O ION FURNACE HEAT Gharpy V-notch impact Tensile properties 2 properties 3 Yield Reduc- Energy Specistrength Elongation of absorption, Fibrous men (0.2 Tensile tion in 2 area, ft.-lb. fracture, percent orientaofiset) strength, inches, per- Plate No. tion l k.s.1. k.s.i. percent cent 0 F. l F. 0 F. -120 F.

1-inch-thiek plate 1%-inch-thiek plate Z-inch-thiek plate 3-ineh-thick plate t-inch-thick plate l '1 is transverse to final rolling direction, and L is parallel to final rolling direction.

2 Results of single test. 8 Average of three tests.

From the foregoing, it can be seen that an improved alloy steel has been obtained having a low carbon content and an as-quenched microstructure containing significant amounts of bainite (up to 100 percent). In spite 50 2,770,563

of the large amounts of bainite, the steels of the present invention, when tempered, exhibit high strength and toughness properties with significantly improved weldability properties over the prior art.

Obviously many modifications and variations of the 5 present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is: t

1. A. low carbon steel plate consisting of from 0.09 to References Cited UNITED STATES PATENTS 

